For a notebook computer, a LCD (Liquid Crystal Display) is usually used as a display device. Because the LCD itself does not have a light source, a cold cathode fluorescent lamp is used to emit light under the control of a backlight converter. The backlight converter comprises a DC/DC power adapter, a pulse width modulator (PWM) and a backlight feedback control circuit. The cold cathode fluorescent lamp is mounted at the output of the pulse width modulator. The backlight feedback control circuit receives a brightness regulation signal from the notebook computer system, and controls the magnitude of the output voltage of the DC/DC power adapter and the frequency of the pulse width modulator.
In the backlight converter, the input voltage VIN is converted from the voltage level in DC/DC power adapter into a high voltage. A high frequency signal is generated by means of the oscillation in the pulse width modulator to form a high voltage having the high frequency for actuating gas within the cold cathode fluorescent lamp to emit light. Because the activity of the gas in the cold cathode fluorescent lamp varies according to the environmental temperature, and because the sale places of the notebook manufacturers probably include Europe, America, Canada, Japan, etc., it is possible that at low temperature the backlight converter can not provide sufficient high voltage and current to allow the cold cathode fluorescent lamp to retain normal brightness. At the worst condition the lamp may not be turned on.